The Shoulder Girdle - Bones
What is the humerus?
The humerus is a long bone of the upper limb, which extends from the shoulder to the elbow.
The proximal aspect of the humerus articulates with the glenoid fossa of the scapula, forming the glenohumeral joint.
Distally, at the elbow joint, the humerus articulates with the head of the radius and trochlear notch of the ulna.
The proximal humerus is marked by a head, anatomical neck, surgical neck, greater and lesser tubercles and intertubercular sulcus (groove).
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The upper end of the humerus consists of the head. This faces medially, upwards and backwards and is separated from the greater and lesser tubercles by the anatomical neck.
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The greater tubercle is located laterally on the humerus and has anterior and posterior surfaces. It serves as an attachment site for three of the rotator cuff muscles – supraspinatus, infraspinatus and teres minor – they attach to superior, middle and inferior facets (respectively) on the greater tubercle.
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The lesser tubercle is much smaller, and more medially located on the bone. It only has an anterior surface. It provides attachment for the last rotator cuff muscle – the subscapularis.
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Separating the two tubercles is a deep groove, known as the intertubercular sulcus. The tendon of the long head of the biceps brachii emerges from the shoulder joint and runs through this groove.
The edges of the intertubercular sulcus are known as lips. Pectoralis major, teres major and latissimus dorsi insert on the lips of the intertubercular sulcus.
This can be remembered with the mnemonic “a lady between two majors”, with latissimus dorsi attaching between teres major on the medial lip and pectoralis major laterally.
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The surgical neck runs from just distal to the tubercles to the shaft of the humerus. The axillary nerve and circumflex humeral vessels lie against the bone here.
The shaft of the humerus is the site of attachment for various muscles.
Cross section views reveal it to be circular proximally and flattened distally.
On the lateral side of the humeral shaft is a roughened surface where the deltoid muscle attaches. This is known is as the deltoid tuberosity.
The radial (or spiral) groove is a shallow depression that runs diagonally down the posterior surface of the humerus, parallel to the deltoid tuberosity. The radial nerve and profunda brachii artery lie in this groove. The following muscles attach to the humerus along its shaft:
Anteriorly – coracobrachialis, deltoid, brachialis, brachioradialis.
Posteriorly – medial and lateral heads of the triceps (the spiral groove demarcates their respective origins).
(For the purpose of this article about the shoulder, we will not be discussing the anatomy of the distal humerus, as it will be covered in the article about the elbow)
What is the scapula?
The scapula (also known as the shoulder blade) articulates with the humerus at the glenohumeral joint, and with the clavicle at the acromioclavicular joint. In doing so, the scapula connects the upper limb to the trunk.
It is a triangular, flat bone, which serves as a site for attachment for 17 muscles.
The costal (anterior) surface of the scapula faces the ribcage.
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It contains a large concave depression over most of its surface, known as the subscapular fossa — the subscapularis (rotator cuff muscle) originates from this fossa.
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Originating from the superolateral surface of the costal scapula is the coracoid process — it is a hook-like projection, which lies just underneath the clavicle.
The pectoralis minor attaches here, while the coracobrachialis and biceps brachii (short head) muscles originates from this projection.
The lateral surface of the scapula faces the humerus.
It is the site of the glenohumeral joint, and of various muscle attachments. Its important bony landmarks include:
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A shallow cavity, located superiorly on the lateral border, which articulates with the head of the humerus to form the glenohumeral (shoulder) joint.
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A roughening immediately superior to the glenoid fossa, which serves as the place of attachment of the long head of the biceps brachii.
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A roughening immediately inferior to the glenoid fossa, which serves as the place of attachment of the long head of the triceps brachii.
The posterior surface of the scapula faces outwards.
It is a site of origin for the majority of the rotator cuff muscles of the shoulder and is marked by:
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The most prominent feature of the posterior scapula. It runs transversely across the scapula, dividing the surface into two.
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Projection of the spine that arches over the glenohumeral joint and articulates with the clavicle at the acromioclavicular joint.
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The area below the spine of the scapula, it displays a convex shape — the infraspinatus muscle originates from this area.
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The area above the spine of the scapula, it is much smaller than the infraspinous fossa, and is more convex in shape. The supraspinatus muscle originates from this area.
What is the Clavicle?
The clavicle (collarbone) extends between the manubrium of the sternum and the acromion of the scapula.
It is classed as a long bone, and can be palpated along its length (In some individuals, it is visible under the skin.
The clavicle has three main functions:
Attaches the upper limb to the trunk as part of the ‘shoulder girdle’.
Protects the underlying neurovascular structures supplying the upper limb.
Transmits force from the upper limb to the axial skeleton.
The clavicle is a slender bone with an ‘S’ shape. Facing forward, the medial aspect is convex, and the lateral aspect concave.
It can be divided into a sternal end, a shaft and an acromial end
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The sternal (medial) end contains a large facet – for articulation with the manubrium of the sternum at the sternoclavicular joint. The inferior surface of the sternal end is marked by a rough oval depression for the costoclavicular ligament (a ligament of the SC joint).
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The shaft of the clavicle acts a point of origin and attachment for several muscles – deltoid, trapezius, subclavius, pectoralis major, sternocleidomastoid and sternohyoid.
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The acromial (lateral) end houses a small facet for articulation with the acromion of the scapula at the acromioclavicular joint. It also serves as an attachment point for two ligaments:
Conoid tubercle – attachment point of the conoid ligament, the medial part of the coracoclavicular ligament.
Trapezoid line – attachment point of the trapezoid ligament, the lateral part of the coracoclavicular ligament. The coracoclavicular ligament is a very strong structure, effectively suspending the weight of the upper limb from the clavicle.
Understanding the Shoulder Joint
The GH joint
The shoulder joint is formed by the articulation of the head of the humerus with the glenoid fossa of the scapula.
It is the major joint connecting the upper limb to the trunk.
Like most synovial joints, the articulating surfaces are covered with hyaline cartilage.
The head of the humerus is much larger than the glenoid fossa, giving the joint a wide range of movement at the cost of inherent instability.
To reduce the disproportion in surfaces, the glenoid fossa is deepened by a fibrocartilage rim, called the glenoid labrum.
The joint capsule is a fibrous sheath which encloses the structures of the joint.
It extends from the anatomical neck of the humerus to the border or ‘rim’ of the glenoid fossa. The joint capsule is lax, permitting greater mobility (particularly abduction).
The synovial membrane lines the inner surface of the joint capsule, and produces synovial fluid to reduce friction between the articular surfaces.
The glenoid labrum is a fibrocartilaginous ridge surrounding the glenoid cavity.
It deepens the cavity and creates a seal with the head of humerus, reducing the risk of dislocation.
The AC joint
The acromioclavicular joint consists of an articulation between the lateral end of the clavicle and the acromion of the scapula. It has two atypical features:
The articular surfaces of the joint are lined with fibrocartilage (as opposed to hyaline cartilage).
The joint cavity is partially divided by an articular disc – a wedge of fibrocartilage suspended from the upper part of the capsule.
The joint capsule consists of a loose fibrous layer which encloses the two articular surfaces. It also gives rise to the articular disc.
The posterior aspect of the joint capsule is reinforced by fibres from the trapezius muscle.
As would be expected of a synovial joint, joint capsule is lined internally by a synovial membrane. This secretes synovial fluid into the cavity of the joint.
The acromioclavicular joint allows a degree of axial rotation and anteroposterior movement. As no muscles act directly on the joint, all movement is passive, and is initiated by movement at other joints
The SC joint
The sternoclavicular joint consists of the sternal (medial) end of the clavicle, the manubrium of the sternum, and part of the 1st costal cartilage.
The articular surfaces are covered with fibrocartilage (as opposed to hyaline cartilage, present in the majority of synovial joints).
The joint is separated into two compartments by a fibrocartilaginous articular disc.
The joint capsule consists of a fibrous outer layer, and inner synovial membrane.
The fibrous layer extends from the epiphysis of the sternal end of the clavicle, to the borders of the articular surfaces and the articular disc.
A synovial membrane lines the inner surface and produces synovial fluid to reduce friction between the articulating structures.
The sternoclavicular joint has a large degree of mobility. There are several movements that require joint involvement:
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Shrugging the shoulders or abducting the arm over 90º
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Drooping shoulders or extending the arm at the shoulder behind the body
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Moving the shoulder girdle anteriorly
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Moving the shoulder girdle posteriorly
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When the arm is raised over the head by flexion the clavicle rotates passively as the scapula rotates. This is transmitted to the clavicle by the coracoclavicular ligaments
What are the synovial bursae?
To reduce friction in the shoulder joint, several synovial bursae are present. A bursa is a synovial fluid-filled sac, that acts as a cushion between tendons and other joint structures.
The bursae that are important clinically are:
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Located deep to the deltoid and acromion, and superficial to the supraspinatus tendon and joint capsule. The subacromial bursa reduces friction beneath the deltoid, promoting free motion of the rotator cuff tendons. Subacromial bursitis (i.e. inflammation of the bursa) can be a cause of shoulder pain.
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Located between the subscapularis tendon and the scapula. It reduces wear and tear on the tendon during movement at the shoulder joint.
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Located in the shoulder joint inferior to the deltoid muscle and superior to the head of the humerus. It plays an important role in decreasing friction in the shoulder joint and protects the surrounding tissues of the joint.
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Located anterior to the subscapularis muscle and inferior to the coracoid process. Its function is to reduce friction between the coracobrachialis, subscapularis and short head of the biceps tendons, thus facilitating internal and external rotation of the shoulder.
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Located on the superior aspect of the acromion
